A technique that forms a nitrogen diffusion layer in a surface layer of a metal material to harden the surface layer is generally known. The technique is referred to as a nitriding treatment. A plasma nitriding treatment using nitrogen plasma is a nitriding treatment capable of forming a nitrogen diffusion layer in a surface layer of a metal material at a lower temperature. However, since nitrogen ions in the nitrogen plasma are excessively supplied to the surface of the metal material in a conventional plasma nitriding treatment (referred to as an ion nitriding treatment), an undesirable brittle nitrogen compound layer is formed in the surface layer of the metal material on top of a nitrogen diffusion layer formed in the surface layer of the metal material (see undermentioned Patent Document 1).
A neutral nitriding treatment has been devised as a plasma nitriding treatment that suppresses the formation of the nitrogen compound layer. In neutral nitriding treatment, the metal material is protected so that nitrogen ions in the nitrogen plasma is prevented from entering the metal material and only nitrogen atoms contribute to the formation of the nitrogen diffusion layer (see undermentioned Patent Document 2).